Why is the Martian sky pink? There are two factors that contribute to the color of any sky. The first is the type of material in the atmosphere of the plant. The second is the amount of material through which you view the sunlight. The type of material can tell you which colors of light are scattered and which are absorbed. More material tends to redden the sunlight. The Earth's blue sky is caused by sunlight scattering off of the nitrogen molecules that dominate the atmosphere. Blue light is more easily scattered by the nitrogen molecules than red light. Because the scattered light comes into our eyes from all directions, it is the scattered blue light that gives the Earth's sky its beautiful color. When the Sun is on the Earth's horizon, the sunlight is reddened because it passes through more atmosphere, and all of the blue light has been scattered out, leaving a reddish-colored Sun. The Martian sky acts in a similar way. The carbon dioxide molecules scatter the light providing a blue background, but there is a critical difference between the type of material in the Earth's sky and the Martian sky. When sunlight enters the Martian atmosphere, it encounters large amounts of dust borne aloft by winds and storms across the planet. The dust is mostly composed of particles of the mineral magnetite. This mineral is known to absorb blue light and scatter red light. Since our eyes perceive the light that is scattered, we see the Martian sky as reddish or pinkish. On clear days, the sky is a more familiar, but darker, blue. On dusty days, the sky is almost rust-red. What's the weather like? The winds do not have much force behind them because the atmosphere is so thin. The skies are usually clear with thin, high, blue clouds. When sunlight hits the Martian plains, it heats the surface dust, making a swirling updraft that sucks dust into the air. These monster dust devils are up to 5 miles high! If solar heating is strong in the summer, it gets hot enough to create dust storms. Some of these local storms become so strong they are thousands of miles across. Every few years, these "great storms" are powerful enough to encircle the entire planet! This means that any buildings built on Mars have to withstand a mild sandblasting on a regular basis that may last for months. Click here for images and movies of Martian dust devils. What makes the wind blow on Mars? Carbon dioxide freezes onto the polar ice cap in the winter and sublimes (changes from a solid directly into a gas) off the polar ice cap in the summer. When the north pole of Mars, for example, is having summer, carbon dioxide (and some water ice) is subliming off and moving away from the pole. Meanwhile, the southern pole is having winter, so carbon dioxide is freezing out of the atmosphere onto the pole. This freezing process creates an area of lower air pressure which is filled by air moving in from the higher pressure region in the north. This process is reversed when the north pole is in winter and the south pole is in summer. The movement of carbon dioxide in this manner creates global wind patterns. This is the south polar cap of Mars as it appeared to the Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) on April 17, 2000. In winter and early spring, this entire scene would be covered by frost. In summer, the cap shrinks to its minimum size, as shown here. Even though it is summer, observations made by the Viking orbiters in the 1970s showed that the south polar cap remains cold enough that the polar frost (seen here as white) consists of carbon dioxide. The polar cap from left to right is about 260 miles (420 km) across. How does the air move? Air rises and travels in large circular motions called "Hadley cells". Earth has three pairs of these cells to move air to the poles and back. However, Mars only has one Hadley cell where warm air rises at the latitude of greatest solar heating, moves poleward, cools, and sinks. This global circulation is complicated by the rotation of the planet, which tugs the atmosphere along with it. This means air traveling north to south is dragged sideways by Mars' spin, causing the wind to blow more west to east. Are there any other hazards on the Martian surface? Nuclear radiation is a part of our natural environment. Certain rock types continuously release radiation from the radioactive elements embedded in them and radiation from space is constantly bombarding the surface of the planet. While we can avoid building near the radioactive rocks, we need protection from energetic particles emitted during solar flares and from the cosmic radiation coming in from the Milky Way Galaxy. The Earth's magnetic field traps a fair amount of the solar and cosmic radiation while the Earth's thick atmosphere helps absorb those particles that do make it through the magnetic field. Even so, on Earth, humans receive the radiation equivalent from cosmic rays of about a half dose of a medical x-ray per year. Mars has no magnetic field and its atmosphere is too thin to stop much of the solar and cosmic radiation. The equivalent dose on Mars for radiation is about 40 medical x-rays per year. The general accepted safe level of radiation exposure is about 3 or 4 medical x-rays per year. On Mars, we must protect ourselves from radiation hazards by making sure our buildings have enough material on their roofs to absorb the radiation. Click here for a Martian weather report taken during the Mars Pathfinder mission.